<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    var info = 'why'
    console.log(typeof info); //string

    var age
    console.log(typeof age) // undefined

    var address = null
    console.log(typeof address) // object

    // ()的作用
    // 1. 调用函数
    // 2. 当成整体
    console.log(typeof (info))
  </script>
</body>

</html>